(1) Field of the Invention
This invention relates to a process for the twin-roll type, continuous casting of metal sheets, which produces cast metal pieces in a sheet form directly from a molten metal.
(2) Prior Art
A process for producing cast metal pieces in a sheet form, which comprises pouring a molten metal into the clearance between a pair of rotating rolls, solidifying the poured molten metal and rolling the solidified metal, is known as Bessemer process. The cast metal pieces obtained according to the process have a thickness of a few millimeters, and are very thin, as compared with the steel ingots and continuously cast slabs produced according to the conventional process, and thus cannot have a higher draft when cold rolled. That is, cast surface state, particularly surface wrinkling and cracking, of cast metal pieces is an important problem. That is, it is important to control the surface state of cast metal pieces with a high precision.
In order to improve the surface state of cast metal pieces, it was attempted to improve a casting nozzle to gently supply a molten metal into the clearance between the rolls and minimize fluctuation at the meniscus, which becomes a cause for the wrinkling at the cast surfaces of cast metal pieces, as shown in Japanese Patent Publication No. 52-23327, etc. However, it is difficult in these prior art processes to completely eliminate the fluctuation of the surface of molten metal at the meniscus and also to flatten the cast surfaces of cast metal pieces.
In order to solve the problem of pouring a molten metal on the other hand, it was attempted to start formation of solidified shell below the meniscus of molten metal, thereby improving the surface state of cast metal pieces, as shown in Japanese Patent Applications Kokai (laid-open) Nos. 61-30260 and 61-186153.
Furthermore, it was also attempted to provide control plates in the pool of molten metal formed between a pair of rolls to adjust the contact area between the molten metal and the rolls and control a position of beginning of a solidification under the surface of the molten metal, thereby rectify fluctuation in the thickness of cast metal pieces and making the surface state of the cast metal pieces good, as disclosed in Japanese Patent Applications Kokai (Laid-open) Nos. 58-148056, 59-33059 and 60-21161, and Japanese utility Model Application Kokai (Laid-open) No. 62-61349.
However, it is difficult in these prior art processes to completely prevent wrinkling or cracking at the surfaces of cast metal pieces under every casting conditions.